Preparation of polynitriles by ammoxidation of polyalkyl substituted aromatic hydrocarbons utilizing recycle of unreacted hydrocarbons and intermediate nitriles



United States Patent 3,497,545 PREPARATION OF POLYNITRILES BY AMMOXI- DATION 0F POLYALKYL SUBSTITUTED ARO- MATIC HYDROCARBONS UTILIZING RECYCLE 0F UNREACTED HYDROCARBONS AND INTER- MEDIATE NITRILES Richard L. Golden, Oradell, and Hyung-Duk D. Yoo, North Bergen, NJ. assignors to Halcon International, Inc, a corporation of Delaware N0 Drawing. Filed Mar. 10, 1966, Ser. No. 533,237 Int. Cl. C07c 121/02, 121/58 US. Cl. 260-465 1 Claim ABSTRACT OF THE DISCLOSURE A process for preparing diand tri-nitriles from alkyl aromatics by ammoxidation, wherein the reaction is carried out such that 20 to 80% of the starting material is reacted and such that mono-nitriles are produced and re-cycled.

The present invention pertains to the production of nitriles from polyalkyl substituted aromatic hydrocarbons. Particularly, the invention is concerned with the improved ammoxidation of a dialkyl aromatic hydrocarbon such as para xylene to the dinitrile product.

Aromatic dinitriles such as terephthalonitrile and isophthalonitrile are chemicals of very considerable importance primarily because these materials can be conveniently converted to aromatic diacids such as terephthalic acid. Such acids, of course, are used in very large amounts in the synthetic fiber field.

The conversion of polyalkyl aromatics to the aromatic nitriles by the simultaneous reaction with ammonia and oxygen, the ammoxidation reaction, is by now a quite well-known reaction. The reaction has heretofore been performed in the vapor phase by passage of the reactants through a fixed bed of catalyst at conditions which insure substantially complete conversion of the hydrocarbon per pass. This process, while advantageous as contrasted with previous routes, results in a nitrile selectivity of the order of 75% or so. Due to hot spotting and general reaction instability it has not been possible to achieve higher selectivities in a consistent, commercial scale. In view of such inefliciencies remaining in the ammoxidation reaction, it is apparent that there is considerable room for further improvement.

It is an object of the present invention to provide an improved method for the ammoxidation of polyalkyl aromatics to polynitrile products.

It is a particular object of the invention to provide a process for the ammoxidation of xylenes to the corresponding dinitrile products.

It is a special object of the invention to provide an improved method for the ammoxidation of para xylene to terephthalonitrile.

Other objects of the invention will be apparent from the following description.

In accordance with the present invention, it has been found that the ammoxidation selectivity can be very markedly improved by the employment of fluidized solid contact techniques, by the regulation of the reaction to provide only a partial conversion per pass and through the provision of the separation and recycle of intermediate mononitrile ammoxidation products. In practice of the present invention, a polyalkyl aromatic hydrocarbon is ammoxidized during contact with a fluidized solid catalyst at ammoxidation conditions such that about 20 to 80% and preferably 30 60% of the polyalkyl aromatic is reacted per pass. The reaction mixture is separated into its various components, polynitrile aro- 3,497,545 Patented Feb. 24, 1970 ice matic comprising the product of the process with the unreacted polyalkyl aromatic and intermediate aromatic mononitrile being recycled to the ammoxidation reaction. The reaction is carried out using continuous processing procedures and overall reaction selectivities of the polyalkyl aromatic to polynitrile aromatic of or higher are readily obtained.

The invention is especially applicable to the ammoxidation of dialkyl aromatic hydrocarbons to dinitriles although included is the conversion of trialkyl aromatics to dinitriles or trinitriles. Preferably, the aromatic compounds have one' aromatic ring and 2 or 3 alkyl substituents, each of which has no more than about 3 carbon atoms. Most preferably, the invention is applicable to the ammoxidation of xylenes and most notably to the ammoxidation of para xylene with the production of terephthalonitrile.

Examples of specific practices of the invention include the ammoxidation of meta xylene to isophthalonitrile, the ammoxidation of mesitylene to the corresponding trinitrile, the ammoxidation of paradiethylbenzene to terephthalonitrile, and the like.

In practice of the invention, the fluidized solid ammoxidation reaction is carried out in continuous fashion. The hydrocarbon together with appropriate amounts of ammonia or an ammonia yielding compound and an oxygen containing gas are fed to the reaction continuously in amount necessary to make up for that proportion reacted per pass. Also fed to the reactor is recycle hydrocarbon as well as mononitrile formed in a previous pass through the reaction zone. These gases are passed through an appropriate ammoxidation catalyst in such a way that the catalyst is kept in fluidized motion resembling a boiling liquid. Most suitable, the overall composition by volume of the gases entering the reaction zone is 1.0 to 3.0% polyalkyl aromatic, 6 to 18% ammonia or ammonia-yielding compound expressed as the equivalent of ammonia, 7.0 to 9.0% molecular oxygen, 1.0 to 2.0% aromatic mononitrile and the remainder inert gases including steam and nitrogen. Since the preferred source of the molecular oxygen is air, the reaction mixture will normally contain nitrogen. It is not necessary but generally preferred to add steam such that the steam comprises up to 15%, and preferably about 0.5 to 2.5% of the total feed to the reaction. Below 0.5% steam conversion drops oif somewhat while above 2.5% steam there is little or no further advantage.

Suitable ammoxidation temperatures are generally in the range of 300 to 800 C. and preferably 425 to 500 C. Substantially atmospheric pressure conditions are satis factory although higher pressures can be employed if desired. The gaseous mixture is continuously introduced into the fluidized solid reaction zone at a rate suflicient to maintain the catalyst influid motion. Illustratively, the rate of gas flow is generally 1.3 gr. mole/cm. hr. to 1.6 g. mole/cm. hr. and preferably 1.4 to 1.5 gr. mole/cm. hr. with catalyst particle size of 60/ mesh.

Catalysts which are employed in the present invention are of the type which are known ammoxidation catalysts. A particularly preferred catalyst is a vanadium and antimony supported on a suitable support. However, other catalysts comprising a compound of vanadium oxide preferably in combination with a tin, chromium, bismuth, platinum or molybdenum oxide can be employed. The catalysts are suitably supported on alumina, silicon carbide, pumice, zirconia and the like. The catalyst most desirably has a particle size between 16 and 325 mesh, preferably between about 40 and mesh.

The ammoxidation reaction is carefully controlled so as to provide for only a partial conversion of the polyalkyl aromatic charged to the reaction zone. This can conveniently be accomplished by skilled persons by adjusting the residence time, temperature and ratio of reactants in order to achieve the desired conversion per pass. Polyalkyl aromatic conversion per pass of about 20 to 80% and preferably 30 to 60% are maintained during practice of the invention. The reaction mixture is separated by conventional techniques. An extremely important advantage of this invention is the fact that the aromatic polynitrile product is recovered in extremely high purity due to the high overall selectivity of this reaction. This is particularly important since the primary use of the product nitrile is in the production of diacids for fiber use; an essential of fiber grade diacids is extreme purity. The unreacted hydrocarbon as well as mononitrile produced as an intermediate is continuously recycled to the ammoxidation wherein the recycle material is admixed with suflicient net fresh ammonia, oxygen, dialkyl hydrocarbon, and steam or other inert gas to maintain substantially constant feed to the reactor.

The following example will serve to illustrate practice of the present invention.

EXAMPLE 1 Paraxylene is ammoxidized to terephthalonitrile in accordance with the invention. The catalyst which is employed is a low surface area alumina powder of 60-140 mesh having impregnated thereon 1.3% by weight V and 8.1% by Weight Sb O The reactor has a two inch inner diameter and a 40 inch length including the disengaging section. The feed to the reactor is vaporized before being introduced into the reactor through a distributor having 97 holes each of 0.03 inch diameter.

The feed rate is 29.5 moles/hr., the reaction temperature is about 454 C., reaction pressure is about 5 p.s.i.g., feed space velocity is about 1100 hrr and the catalyst bed expansion is about 24% Including recycle of the mononitrile (p-tolunitrile), unreacted p-xylene, and nitrogen the composition of the total vapor feed to the reactor on a mol percent basis is about 2.5% p-xylene plus moronitrile, 1.3% H 0, 14.0% NH and 82.2% oxygen plus nitrogen (of which 8.3% by volume is oxygen). Ammonia, xyene, and air are added to recycle materials at the rate needed to form the above total feed.

During the reaction about 50% of the xylene is reacted per pass. The reaction mixture is withdrawn from the reactor, and cooled to about 65 C.

The condensed product comprises on a mol basis about 29.0% terephthalonitrile, about 38.0% p-tolunitrile, about 3.2% other materials, and about 29.8% unreacted pxylene.

The condensate is distilled in a first distillation and p-xylene and p-tolunitrile are taken over head and re cycled with a portion of the nitrogen to the ammoxidation.

The crude terephthalonitrile is distilled at atmospheric;

pressure and an overhead temperature of 285 C. and product terephthalonitrile is recovered overhead.

The overall yield of terephthalonitrile is 90% based on the net paraxylene fed to the system.

The product terephthalonitrile is readily converted to high purity terephthalic acid as by refluxing in aqueous sodium hydroxide with ammonia removal and subsequent acidification as with aqueous dilute sulfuric acid.

EXAMPLE 2 The procedure of Example 1 is repeated except that meta xylene is converted to isophthalonitrile and similar results are obtained.

EXAMPLE 3 The procedure of Example 1 is repeated except that p-diethyl benzene is converted to terephthalonitrile and similar results are obtained.

What is claimed is:

1. A process for preparing terephthalonitrile which comprises the continuous oxygen ammoxidation in the vapor phase at a temperature between 300800 C. of paraxylene, said ammoxidation being conducted with a fluidized solid catalyst selected from the group consisting of vanadium oxide, antimony oxide and combinations of vanadium oxide with tin, chromium, bismuth, platinum or mloybdenum oxide on a support selected from the group consisting of alumina silicon carbide, pumice and zirconia; maintaining the conversion per pass of paraxylene in the range of 30 to and continuously recycling unreacted paraxylene as well as intermediate nitrile formed during a previous reaction pass to the ammoxidation reaction, the ammoxidation being carried out such that the overall yield of terephthalonitrile is at least based on paraxylene.

References Cited UNITED STATES PATENTS 2,496,661 2/1950 Denton et al. 260-465 X 2,833,807 5/1958 Farkas et al. 260465 2,838,558 6/1958 Hadley et al. 260-465 FOREIGN PATENTS 619,842 5/1961 Canada.

512,864 5/1955 Canada.

645,754 11/1950 Great Britain.

956,892 4/ 1964 Great Britain. 1,141,274 12/ 1962 Germany.

CHARLES B. PARKER, Primary Examiner S. T. LAWRENCE III, Assistant Examiner U.S. Cl. X.R. 

